The present application generally relates to systems and methods for communicating high speed signals between a transceiver and a processing module in a communication device, such as an optical line termination.
In an optical line termination, an optical transceiver receives an optical signal modulated with a data stream from an optical network unit and converts the optical signal to an electrical signal. The electrical signal from the optical transceiver is provided to a processing module for further processing as may be desired. Frequently, the optical transceiver and the processing module are provided in separate circuits (possibly in separate cards or separate integrated circuits) having different DC (direct current) offset voltage level or common mode voltage level requirements. For example, the optical transceiver can output a signal having a DC offset voltage or common mode voltage (e.g., 3.3 V) that is significantly greater than the DC offset voltage or common mode voltage (e.g., 1.2 V) for the signal that can be received by the processing module. Thus, the electrical signal from the optical transceiver cannot be provided directly to the processing module because the processing module is not equipped to handle the signal with the higher DC offset voltage or common mode voltage.
For compatibility between the optical transceiver and the processing module, the DC offset voltage or common mode voltage for the signal from the optical transceiver has to be level shifted so that the signal can be received by the processing module. One way to couple the optical transceiver to the processing module to obtain the desired level shift is with a resistive divider. However, a drawback of the resistive divider is that the resistive divider discards a significant amount of the signal being communicated between the optical transceiver and the processing module. Another way to couple the optical transceiver to the processing module to obtain the desired level shift is with capacitive coupling. One drawback to capacitive coupling is that the coupling capacitors do not provide an appropriate DC response if there are long idle times or long strings of “1s” or “0s” in the data stream. Still another way to couple the optical transceiver to the processing module to obtain the desired level shift is with bus transceivers. A drawback to the use of the bus transceivers is that they undesirably introduce jitter into the signal.